Method for forming on workpiece resilient thread having closed helical cavity inside the thread

ABSTRACT

A method of forming on a workpiece a resilient thread, having a closed helical cavity inside the thread, includes plastic deformation of the walls of a preformed helical groove whose depth exceeds the depth of the roots of the thread to be obtained. The volume of the workpiece material, expelled in forming the helical groove, correspondsto the volume of the closed helical cavity inside the thread being obtained. 
     A thread-forming tool for carrying out the method comprises a shank and an operating piece which consists of a groove-forming portion and a thread-forming portion arranged in series along one axis and having the same pitch of projections of their thread. The groove-forming portion and the thread-forming portion are displaced relative to each other in the axial direction at least by half the pitch, the major diameter of the groove-forming portion exceeding the major diameter of the thread-forming portion.

FIELD OF THE INVENTION

The invention relates to the art of metal working by pressure, and moreparticularly it concerns methods and tools for forming on a workpieceresilient threads having a closed helical cavity inside the thread byplastic deformation of the metal in a cold state.

DESCRIPTION OF THE PRIOR ART

Methods of forming threads by plastic deformation of the metal of aworkpiece, by rolling in particular, which have wide application inmodern practice (see, for example, Pisarevsky M. I., Nakativanietochnikh rezb i shlitsev, Moscow-Leningrad, Mashgiz, 1968), togetherwith such obvious advantages as high productive capacity, metal savingand high precision of resulting threads, are characterized bysubstantial disadvantages. Among these are, first of all, a stricttolerance for the size of a workpiece to prevent jamming of the toolwhen forming a thread in bores, and considerable thread-forming forcesacting on the tool, which is particularly unwanted when forming a threadin bores of small diameter (less than 8 mm).

Known in the art is a method of forming a resilient thread having aclosed helical cavity inside the thread disclosed in Japan Pat. No.48-20,977 published June 25, 1973.

The above method comprises performing a helical groove in a workpieceand subsequent plastic deformation of its walls by means of athread-forming tool. As noted in the patent specification, the helicalgroove is formed so as to have a depth in the range of 1/2 of the depthof thread to the complete depth of thread, but not in excess.

To form a thread, the thread-forming tool is deepened in the tops of thehelical groove deforming its walls, which results in obtaining a threadhaving a closed helical cavity inside the thread.

The thread thus obtained, by virtue of the closed helical cavity insidethe thread, allows widening of the tolerance for the size of aworkpiece, reduction of the thread-forming forces acting on the tool andelimination of jamming of the tool in the operation of rolling a femalethread.

However, when deforming a helical groove having a depth in the range ofhalf the depth of the thread being obtained to its complete depth butnot in excess, the future cavity is positioned at the tops of the threadand is limited by the size of its projections. Moreover, the mostintensive deformation of the workpiece material under the action of thethread-forming portion of the tool takes place at the tops of thethread, and the cavity turns out to be narrow. It prevents thesubstantial increase of the thread resilience, widening of the range oftolerance for workpiece size, and reduction of the torque in therotation of the thread-forming tool in the operation of forming athread.

Known in the art is a threading tool-screw tap for rolling in boresresilient thread having a closed helical cavity inside the thread as inUSSR Inventor's Certificate No. 625,824 published Sept. 30, 1978.

This screw tap comprises a groove-forming portion and a thread-formingportion arranged in series and rigidly connected with each other.Projections of the thread of both portions have the same pitch and aredisplaced along the axis of the screw tap at least by half the pitchrelative to each other, the major diameter of the projections of thegroove-forming portion not exceeding the major diameter of thethread-forming portion.

To obtain a resilient thread having a closed helical cavity inside thethread, the screw tap is screwed in a bore. In doing so, the first(groove-forming) portion forms a groove having a depth within the rangeof half the depth of the thread being obtained to its complete depth. Insubsequent motion of the screw tap, the thread-forming portion followingthe groove-forming portion deepens in the tops of the helical groove anddeforms its walls, which results in forming a thread having a closedhelical cavity inside the thread.

As in the above-described method, the volume of the cavity in the threadobtained by this screw tap is limited by the size of the projections ofits thread because the helical groove formed by the groove-formingportion has a depth not exceeding the complete depth of the thread beingobtained, the cavity being positioned in the place of the most intensivemetal deformation caused by the action of the thread-forming portion andtherefore having a configuration stretched along the axis of the thread.All this, as noted above, prevents the substantial increase of thethread resilience, widening of the range of tolerance of workpiece size,and reduction of the torque in the rotation of the screw tap.

SUMMARY OF THE INVENTION

The invention is a method of forming a resilient thread having a closedhelical cavity inside the thread and a thread-forming tool which, bymeans of displacing the cavity deeper in the thread profile of thethread being obtained, allows increasing the resilience of this thread,widening of the range of tolerance for the size of a workpiece and alsoreduces the torque in rotation of the tool in the process of forming athread.

The problem is solved by a method of forming on a workpiece a resilientthread having a closed helical cavity in the thread by plasticdeformation of the walls of a preformed helical groove. According to theinvention, the helical groove is formed having a depth exceeding byvalue the depth of the root of the thread being obtained, and the volumeof the workpiece material expelled in forming the helical groovecorresponds to the volume of the closed helical cavity inside the threadbeing obtained.

With this method of forming a thread, the roots of the preformed helicalgroove will be deeper than the roots of the future thread, and thecavity being obtained in the operation of plastic deformation of wallsof the helical groove will be displaced deeper in the thread profile ofthe thread being obtained. Furthermore, the cavity is remote from theplace of the most intensive deformation of the workpiece material takingplace under the action of the thread-forming tool, and therefore itsshape and volume make it possible to considerably increase theelasticity of the thread, to widen the range of tolerance (a possibilityappears to obtain a thread on workpieces whose size is not adequate toform a thread by known in the art methods) and to reduce the torque inrotation of the tool in the process of forming a thread.

The problem is also solved by a thread-forming tool for carrying out themethod, comprising a shank and a threading head consisting of agroove-forming portion and a thread-forming portion arranged in seriesand having equal pitches of the projections of their thread, which aredisplaced along the axis of the tool relative to each other at least byhalf the pitch. According to the invention, the major diameter of thegroove-forming portion exceeds the major diameter of the thread-formingportion.

The groove-forming portion is preferably positioned in advance of thethread-forming portion at the side of the shank, the thread-formingportion having a taking section at the side of the groove-formingportion.

In a modification of the thread-forming tool, the groove-forming portionand the thread-forming portion are each made in the form of rollersuniformly positioned on the circumference of a body and having anannular thread equal to the pitch of the thread to be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be explained with reference to certainembodiments thereof and the accompanying drawings, wherein:

FIG. 1 diagrammatically illustrates the process of plastic deformationof the walls of the preformed helical groove, according to theinvention;

FIG. 2 is an elevational view of the thread-forming tool (screw tap)according to the invention;

FIG. 3 diagrammatically illustrates a modification of the screw tapwherein the groove-forming portion is positioned in advance of thethread-forming portion at the side of the shank;

FIG. 4 diagrammatically illustrates a modification of the thread-formingtool with rollers, according to the invention;

FIG. 5 is an end view of the device shown in FIG. 4, viewed from thestart portion of the tool.

DETAILED DESCRIPTION OF THE INVENTION

Forming a thread according to said method is carried out as follows.

On a workpiece 1 a helical groove 2 having a pitch equal to the pitch ofthe thread to be obtained and a depth exceeding the depth of the root ofthe thread profile of the thread to be obtained is preformed, and thevolume of the workpiece material expelled in forming the helical groove2 should correspond to the volume of the closed helical cavity 3 insidethe profile of the thread to be obtained by subsequent deforming of thewalls of this helical groove 2.

An additional volume of the material, expelled in the operation offorming a groove having a depth exceeding that of the root of a threadto be obtained, makes it possible to widen the range of tolerance forthe size of a workpiece and to form a thread, for example, in boreswhose diameter is in the range of the angle diameter to the majordiameter of a thread to be obtained. Moreover, this method may beutilized for restoration of a stripped thread whose remained portion is20 or less percent of the original one (80% wear).

It should be noted that the cavity being obtained in the thread profileof the thread being formed by this method has a larger volume than thatin the known resilient threads, it is remote from the place of the mostintensive deformation of the material caused by the action of thethread-forming portion of the tool and therefore imparts largerelasticity to the thread being obtained and reduces the torque in theprocess of forming a thread.

A thread-forming tool carrying out this method, for example a chiplesstap, comprises a shank 1 (FIG. 2) and a threading head 2 consisting of agroove-forming portion 3 and a thread-forming portion 4 arranged inseries and having equal pitches of the projections of their threads,which are displaced along the axis of the tool relative to each other atleast by half the pitch, the major diameter d₁ of the groove-formingportion 3 exceeding the major diameter d₂ of the thread-forming portion4. According to one embodiment, the groove-forming portion 3 ispositioned in advance of the thread-forming portion 4 at the side of theshank 1 (FIG. 3), the thread-forming portion having a taking section 5at the side of the groove-forming portion.

As shown in FIG. 4 and 5, the groove-forming portion 3 and thethread-forming portion 4 in the thread-forming tool may be each made inthe form of respective rollers 7 and 8 uniformly positioned on thecircumference of a body 6 and having an annular thread equal to thepitch of the thread to be obtained.

A modification of the thread-forming tool is shown in FIGS. 4 and 5where the groove-forming portion 3 with the rollers 6 is positioned inadvance of the thread-forming portion 4 with the rollers 7 at the sideof the shank 1. It is understood that a modification is possible,according to which the portions 3 and 4 with rollers 6 and 7 will bepositioned in the back order, i.e. the portion 4 with the rollers 6 willbe positioned in advance of the portion 3 with the rollers 5 at the sideof the shank 1.

The formation of a thread in a bore is carried out in the following way.

The groove-forming portion 3 (FIG. 2) moving in the bore of a workpiecein an axial direction forms a helical groove by raising a portion of theworkpiece material from the area beyond the diameter of the roots of thethread to be obtained and thus reducing the diameter of the bore in theworkpiece. This done, the thread-forming tool portion 4, whose threadtops are displaced relative to the thread tops of the groove-formingportion 3 at least by half the pitch, comes into operation. By virtue ofthis displacement, the first projection of the thread profile of thethread-forming portion 4 agrees with the projection of the helicalgroove at the moment when the thread-forming portion comes intooperation. Deepening into the tops of the helical groove profile atfurther screwing in of the tool, the thread-forming portion deforms itswalls, which results in obtaining a thread having a closed helicalcavity disposed below the roots of this thread.

When the position of the portions 3 and 4 is interchanged (FIGS. 3 and4), the process of forming a thread will be carried out in the followingway.

The thread-forming portion 4 moves in the bore of a workpiece in theaxial direction freely if the diameter of the bore in the workpiece islarger than the major diameter of the thread to be obtained, or by themethod of self-engagement if the diameter of the bore is smaller thanthe major diameter of the thread to be obtained, which results in theformation of a thread having a small depth. The groove-forming portion3, following the thread-forming portion 4, forms a helical grooveraising a portion of the material from the area beyond the diameter ofthe roots of the thread to be obtained and thus reducing the diameter ofthe original bore.

After reversing the rotation of either the tool or the workpiece, thetool starts to screw out with the groove-forming portion 3 serving as athread template, which directs the tops of the thread of the portion 4strictly between the roots of the obtained groove.

When the screw tap is completely screwed out, the helical groove isturned into a cavity disposed inside the obtained thread. Thus, withscrewing the tool in, there is formed a helical groove in the workpiece,and with its screwing out, a resilient thread is formed.

This tool proves most advantageous in forming a resilient thread inblind bores.

Apparently, in the case when the thread-forming portion 4 and thegroove-forming portion 3 are made in the form of rollers 7 and 8uniformly positioned on the circumference of the body 6 and having anannular thread whose pitch is equal to the pitch of the thread to beobtained, the tool will operate in the same way as it was describedabove.

Now, there is a particular example of the carrying out the method of theinvention.

EXAMPLE

The example of carrying out the method is presented for restoration of astripped thread M 18×1.5 in a steel 20 by means of a two-portionchipless screw tap with the following geometry of the firstgroove-forming portion of the threading head:

profile angle: β=50°;

major diameter: d=18.9 mm;

the value of dulling the top of the profile of the groove-formingportion S=0.2 mm.

The second portion of the threading head is made according to the USSRState Standard 18 842-73.

The minor diameter of the thread which was remained on nuts was 17.8+0.1(75% wear). A resilient thread having a closed helical cavity inside thethread was formed in these nuts with their standard dimensions on alathe (type 1A616) with a single stroke of the screw tap at the speed ofthe lathe spindle 400 r.p.m. A lubrication MP-1 was used.

A statistic research was made on the static strength of the restoredthread under the conditions when the material of a bolt was considerablystronger than that of a nut.

The static strength of the restored thread is higher than the strengthof the thread before the wear by a factor of 1.9.

It is understood that such method of forming a thread may be utilizedalso for forming external threads.

While the invention has been particularly shown and described withreference to the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes and additions may be madewithout departing from the spirit and scope of the invention defined inthe appended claims.

INDUSTRIAL APPLICABILITY

The invention may prove most advantageous in forming a resilient threadin bores whose diameter is between the angle diameter and the majordiameter of a thread being formed. The invention may prove equallyadvantageous in restoraton of stripped thread whose remained portion is20 or less percent of the original one (80% wear).

We claim:
 1. A method of forming a resilient thread having a closedhelical cavity inside the thread on a workpiece, comprising forming ahelical groove having a depth exceeding by value the depth of the rootsof the thread to be obtained, the volume of the material of theworkpiece, expelled in forming the helical groove, corresponding to thevolume of the closed helical cavity inside the thread being formed; andforming said resilient thread by plastic deformation of the walls ofsaid helical groove, wherein the closed helical cavity is disposedwithin the threads and below the roots of said thread.